bungee

Coming from someone who has "cartwheeled" plenty of bungee launches "imodel" is right on and also not too tight on the bungee or you will get gymnastics (like me) not a launch...but really the landing gear is no way to do it. Follow these instruction about installing a hook. Get a small block of wood and drill small hole deep. take 3 inches of hard coat hanger wire (the thick kind) or similar and drill hole for it in fuse at designated location(see posts above) glue (epoxy only) block inside fuse and push coat hanger wire through hole drilled in fuse into hole in block of wood. bend wire as hook. done. Dont try to glue the block of wood on the outside, it will just rip off the fuse. Believe me...

Your tow hook should be half way between the nose and the CG. If the tow hook is too far forward, you get a very shallow angle of attack. The opposite is true if the tow hook is too far back.

actually, according to Nick Ziroli, the hook should be about 2/3 distance between the nose and the balance point. the way we used to do it on gliders, was to turn it upside down and hang it at various points along the fuse. the best position is acheived when the glider hangs almost level with a slight nose down attitude. for all intents and purposes, this postition is about 2/3 between the nose and the balance point (CG). hope that helps you out.

Rich, tow hook position and cat launch position is very different. A tow position allows level pull, no climb. What imodel says is correct place to start. Hanging the jet with the nose up, the jet should have approx 15deg pos away from the zip line. Just about all of my jets are approx 50-60% back from the nose to the CG.

Tape a hook in the approx position and hang as discribed. Adjust to suit. Mount perm. If you have a backing of 1/4" ply, inside the fuse, a 3/16" carbon dowel works great, epoxied in. A small tab of plastic or thin ply at the base of the hook allow protection from the pull ring.

Fuzz

Images

Yep, Fuzz got it right. You need to position the hook so that the inertia of the model makes it rotate to the correct angle of attack when the bungee is accelerating it. This will depend on the 3D position of the CG, not just the longitudinal position of it, as a high CG will tend to rotate the model more than a low CG. If the hook is too far back the model will tend to rotate to too high an angle of attack, causing the wing to be stalled no matter how fast the launch is (remember, wings stall at a given angle of attack, not airspeed), with the result the model does the dreaded bungee death roll.

Hey guys, I am new to bungee launching and am going to be trying it soon. I was wondering how you determined the amount of pull on the cord? I've heard before that it was about 5lbs of pull for every 1lb the plane weighs. Is this correct?
Thanks

Fuzz, I totally agree with you. what I disagree with is the fact that that is where I used to put the hook for bungee launching a glider. if you are using up to 60% on your plane, is that not close to 2/3? if you look at Nick's Turbinator that is designed for bungee launch, he puts the hook at 2/3 between nose and CG. I'm not saying you are wrong at all, but the man is a God in the RC world and I think I'm more inclined to go with him here.

for a more technical reason, we can go further............since we are talking about moments of inertia, too far forward and it's gonna give you a nose down attitude on launch. too far back and you get too nose high. either of those and you are facing a crash with every launch. 60-63% (2/3) gives the best happy medium................in other words, it's gonna give you a bit of nose high at launch, without getting too high in A0A.

there's obviously a reason why Nick chose that hook position with his Turbinator, right? and the reason is, is because it's the most aerodynamically sound position to place it.

Rich, the reason I don't like the "2/3 from the nose" or similar method is that it depends on the length of the nose. What if it's an F5 or F104? Glider launching is a totally different ball game. It happens much slower and you're on the line for a lot longer. A glider launch is designed to get height, whereas a jet launch is designed to get speed.